Medical diagnostic imaging systems employing x-ray and television commonly consist of several major elements An x-ray source propagates x-rays along a beam path. An image intensifier tube is spaced from the source a sufficient distance to allow a patient to be interposed between the source and an input face of the image intensifier tube. The image intensifier tube responds to patterns of x-rays emergent from the patient and incident on its input face to produce at an output face a relatively small and bright visible light image corresponding to the pattern of x-rays emergent from the patient and incident on the input face.
An optical apparatus, sometimes called an "optical cube" is positioned to receive the light from the image appearing at tube output face. The optical cube often includes a lensing system and/or a beam splitter to apportion or selectively direct the light among different optical ports for imaging in one or more of a variety of modalities.
Two such modalities include a cine camera and a television camera. When employed, each of the cine camera and television camera is positioned to view a different optical output port of the optical cube. The cine camera is a device which records moving images on film, analogous to a movie camera.
The television camera is of a known type, and employs a television camera pickup tube. The television camera pickup tube includes a target on which light from the image transmitted from the optical cube falls, and an electron gun to generate an electron beam. Circuitry and apparatus provide for causing the electron beam to scan the target in raster fashion. The television camera, through variations in the electron beam current, produces an electrical video signal which describes the brightness distribution of the image incident upon the target.
The television camera also includes other circuitry for producing appropriate horizontal and vertical television synchronization signals, which, with the video, define the image electronically.
The television camera is also provided with circuitry for automatically adjusting the television camera video signal gain.
The video and synchronization signals, together making up a television signal ensemble, are transmitted to a television monitor which converts the television ensemble to a second, larger visible light image corresponding to the light image produced at the output face of the image intensifire tube. The image produced by the monitor is intended to be of sufficient size, resolution and general quality to serve as a medical diagnostic tool.
The image on the television monitor, in the fluoroscopic system thus far described, is a real time image corresponding o that appearing at the output face of the image intensifier tube. Additionally, the monitor image is useful as a "cine verification", which is the presentation of a television monitor image corresponding to an image being recorded on cine film, to enable an operator to evaluate in real time the quality of the image being recorded on the film for later use.
Imaging systems such as those described here are sometimes equipped with a "brightness stabilization" feature. In brightness stabilization, a light sensor, often a photomultiplier tube, is positioned to view the image of the output face of the image intensifier tube. The photomultiplier tube generates an electrical signal which represents the overall brightness of the image tube output. This signal is coupled to circuitry controlling the x-ray source to adjust the source to maintain optimal brightness of the image at the image tube output. When a photomultiplier tube senses a decrease in brightness at the output face, the source is adjusted to increase its x-ray output and hence increase the brightness of the output image. When an increase is sensed, the x-ray tube is adjusted to reduce x-ray output.
Brightness stabilization has limits in its effectiveness, due to the fact that, irrespective of image brightness, x-ray dose to the patient must be limited to a predetermined maximum. Thus, brightness stabilization is not always able to compensate entirely for a decrease in image brightness.
In operating modes such as described above, it is also desirable to control the gain of the television camera such that the video input signal level, which represents the brightness of the image viewed by the television camera, meets the particular input requirements of the television monitor to optimize monitor picture quality.
There are two methods of controlling television camera gain: fixed gain and automatic gain control, (mentioned above) the latter of which is sometimes known as "AGC".
In fixed gain operation the television camera is preset to have a uniform gain for a given operating mode. Differing fixed gains, however, are sometimes used in accordance with which of a plurality of different operating modes is selected.
Often, fixed gain operation is satisfactory, because the x-ray system, including the x-ray source, is usually operated, during a single study, to provide a relatively constant detected x-ray intensity, which usually yields a relatively constant light intensity of the image appearing at the image tube output face.
Brightness stabilization, as described above, assists in maintaining uniform image tube image brightness.
When the detected x-ray output from the source remains relatively constant, so does the brightness of the light image appearing at the image tube output face. In such circumstances, fixed gain control can yield adequate operation. There are times, however, when the constant light level at the image tube output cannot be maintained.
The major disadvantage of fixed gain operation is that it requires a constant input brightness level from the visible light image at the image tube output.
The brightness delivered to the television camera is approximately proportional to the x-ray intensity emergent from the patient. Thus, a constant brightness corresponds to constant x-ray level from the patient. Government regulations, however, limit the amount of radiation which may be input to a patient's body. With radiation dose being limited, it is not always possible to achieve the desired level of x-rays emergent from the patient to obtain the desired uniform brightness at the image tube output face if there are large amounts of tissue to be traversed by the x-rays. Such large amounts of tissue are regularly encountered when imaging heavy patients, or with radiological views requiring the passage of x-rays through the patient's body at an oblique angle. In such cases, brightness at the image tube output face is often reduced to the point at which the brightness delivered to the television, and consequently the monitor image, is insufficient, and this lack of brightness cannot be overcome by the administration of a higher x-ray dose.
Automatic gain control (AGC) circuitry and technique have sometimes been applied to the television camera to add facility in controlling the level of the television video output to the monitor. In practice of AGC, the television gain is adjusted automatically so that the output to the monitor remains at a specified level notwithstanding brightness changes in the image appearing at the intensifier tube output face. To do this, circuitry is employed which samples the video signal output voltage corresponding to a specified sample window of the intensifier output tube image. Often, the sampled region constitutes a circular central portion of the output image.
The AGC circuitry can be one, or a combination of two kinds. One type represents the average output level within the sampled window. Another type represents the peak brightness level sensed within the sampled window.
The AGC circuitry includes feedback control circuitry operable on the television camera for adjusting the gain of the television camera if the video voltage-deviates from a predetermined optimum reference level. If the video signal rises above the optimum level, the AGC circuitry reduces the video gain of the television camera. If the video signal output voltage falls below the predetermined optimum level, the AGC circuitry increases the video gain of the television camera.
While obviating some of the problems associated with fixed gain, AGC circuitry has its own shortcomings. Through its feedback circuitry, the automatic gain control circuitry is intended to assure that the output voltage, and hence the monitor brightness, remains at the desired level at all times.
A problem with automatic gain control circuitry operation relates to properly specifying the sample window of the intensifier tube output image so that it includes the anatomy of interest and avoids anatomy that is of no particular interest in the study being conducted. This problem is most acute in cardiac cineangiography, where the television system produces a relatively dark image of the heart (the anatomy of interest) directly beside a relatively bright lung image field (in which there is no interest).
There are views in cineangiography where it is virtually impossible to keep the sample area from including significant amounts of the bright adjacent lung field. When the AGC circuitry senses light from the bright lung field, it responds by adjusting downwardly the television camera video gain and making an even darker heart image. In such an image, the heart may not be displayed with enough brightness to be seen, even though the overall sampled area window may have the desired brightness.
There are numerous other examples where the automatic gain control sample window will contain anatomy that is not of interest, but which registers a brightness value which impairs the operation of the automatic gain control circuitry in the manner described above.
It is therefore a general object of this invention to provide an x-ray imaging system which enjoys the advantages of both fixed video gain and automatic video gain control, while reducing or eliminating the disadvantages of both.